1. Field of the Invention
The invention relates to power semiconductor devices, such as insulated gate bipolar transistors (IGBTs), diodes and thyristors. In particular, the invention relates to improving performance of such devices through reduction of lifetime (also known as “lifetime killing”).
2. Related Art
An IGBT 10 as generally known in the art is shown in FIG. 1. The IGBT 10 has collector electrode 11 connected to a p+ collector region 12. Above collector region 12 is n+ type buffer layer 13 and n type base region 14. A pn junction is formed at interface A between collector region 12 and n+ buffer layer 13. Typically, p+ collector region 12 is formed in the semiconductor wafer, while n+ buffer layer and n base region are epitaxially grown thereon. Base or channel diffusion regions 16, 16′ of p material are formed at the upper surface of n region 14, forming p bases, as explained further below. A pn junction is thus formed at the interfaces B, B′.
(As seen, there is an array of diffusion regions in the upper surface of n region 14; one such adjacent region in the array is labeled with the same reference numbers including a prime designation. Additional analogous structure in the adjacent regions is also designated with a prime reference number.)
Within p-type diffusion region 16 is n+ diffusion region 18. (There appear to be two separate n+ diffusion regions in the cross-sectional view of FIG. 1; however, the n+ diffusion region 18 is actually a closed loop within p diffusion region 16 when viewed from above when using a cellular base geometry as shown in U.S. Pat. No. 5,661,314, for example.) An emitter electrode layer 20 contacts the central portion of p diffusion region 16 and an interior portion of n+ diffusion region 18.
A gate electrode 22 extends between the outer portion of n+ diffusion region 18 and the outer portion of n+ diffusion region 18′ of the adjacent diffusion region. Gate electrode 22 is interposed between the-surface of the semiconductor device 10 and the emitter layer 20 and is isolated from the surface of the semiconductor device 10 and the emitter electrode layer 20 by a conventional gate oxide 24 and a low temperature oxide (LTO) 24′.
The IGBT of FIG. 1 operates in a forward mode by applying a positive voltage at the collector electrode 11 with respect to the emitter electrode 20. When gate 22 is biased to a voltage above the threshold voltage VT, an inversion of p base region 16 occurs at the region I. A path is thus formed for electrons through emitter 20, n+ diffusion region 18, the inverted portion of p base region 16 and into n base region 14 and n+ buffer layer 13. The pn junction A between this effective composite n region and the p+ region 12 is forward biased, thus providing a forward conducting state.
When the collector and the emitter are reversed biased, current flow is blocked because the pn junction at interface A is reversed biased.
An important operational parameter for many semiconductor devices, such as the IGBT described above, includes the switching speed or turn-off time. For example, when the device of FIG. 1 is operating in its saturated on condition, there are a large number of minority carriers in the n+ and n-regions 13, 14 and p+ region 12. This concentration of holes and electrons, respectively, must be removed before the transistor 10 returns to its off condition.
There are a number of known techniques for reducing the lifetime of minority carriers in such semiconductor devices. Thus, additional recombination centers in one or more of the regions of the semiconductor device may be provided. An increase of the recombination centers may adversely affect other important operational parameters of the device, such as the forward voltage drop Vce.
One such technique for reducing carrier lifetime is electron irradiation of the device. The irradiation creates lattice defects in the crystal, which act as recombination centers of minority carriers. Because there is a relatively high level of control over the energy, positioning and profile of an electron beam, the degree of damage to the lattice is relatively accurate. Damage created by electron irradiation can be uniformly distributed throughout the silicon, or can be limited to particular sites. A disadvantage of electron irradiation is that the damage to the lattice anneals out at relatively low temperatures, thus reducing the effectiveness of the lifetime killing. The degree of annealing can be affected by other factors related to manufacturing the device. Also, silicon devices subjected to this technique of electron beam (E-beam) radiation demonstrate higher reverse current leakage at elevated temperatures. Further, electron irradiation normally acts uniformly over the full lateral width of the device.
A similar technique of lifetime killing uses particle beam implantation. The lattice damage created by a particle beam is also highly accurate and controllable. The position and degree of the lattice defects are dependent on the size, mass and implant energy of the particle used, among other factors. Thus, the position of the lattice defects can be localized to a particular depth and profile in the silicon. Also, multiple recombination centers can be positioned at different locations throughout the device. However, like E-beam irradiation, damage created by low doses of particle beam implants also anneals out of the silicon at relatively low temperatures; also, devices implanted with particle beams have higher reverse current leakage at elevated temperatures.
Another and very common technique of lifetime killing introduces recombination centers into the silicon through diffusion of heavy metals, such as gold or platinum. Typical heavy metal diffusion temperatures are between 600 and 1000° C. The diffusion temperature controls the solid solubility of the metal atoms in the silicon and thus the density of the impurities. Consequently, lifetime decreases with higher diffusion temperatures. Because of the nature of the heavy metal recombination centers, in many cases the devices have superior characteristics to those processed with electron irradiation or particle beam implants. Also, the recombination centers created by metal diffusion do not anneal out at the relatively low temperatures, as in electron irradiation or particle beam implants. However, heavy metal diffusion is a difficult process to control. Small variations in the processing conditions and/or the silicon used (for example, the substrate doping, manufacturing temperatures, etc.), can create a substantial variation in the lifetime killing, current amplification, forward voltage drop and other characteristics of the semiconductor device.
In general, techniques of fabrication of IGBTs such as that shown in FIG. 1 are known in the art. Materials used in such fabrication are also known. Further, determining and administering electron irradiation or particle beam implantation to a particular semiconductor device in order to create recombination centers at particular regions which reduce lifetime but do not render other operational characteristics unacceptable is either known or can be determined through developed techniques. (For example, U.S. Pat. No. 5,661,314 describes fabrication of an IGBT having certain structural features for improving the packing density and increasing latch current. Use of electron irradiation, or, alternatively, heavy metal diffusion, for reducing lifetime is also described.) Also, determining and administering the appropriate conditions to improve the lifetime of a device by creating recombination centers through heavy metal diffusion (without rendering other operation parameters unacceptable) is also known or can be determined through developed techniques. As noted above, this includes appropriate placement of the regions of lattice defects or heavy metal diffusion to reduce lifetime while maintaining acceptable current amplification and forward voltage drop.
While both contribute to lifetime killing, as noted above the performance characteristics of recombination centers comprised of heavy metal diffusion and recombination centers created via electron irradiation or ion implantation are different. Thus, U.S. Pat. No. 5,747,872 teaches using both types of recombination centers in the same device in order to achieve soft switching and reduction of dynamic avalanche effects over a wider temperature range. The recombination centers provided by the heavy metal diffusion contribute to reduction of carrier lifetime that avoids the dynamic avalanche effect at lower temperatures, but causes large switching power losses (which contributes to dynamic avalanche effects) at higher temperatures. The recombination centers provided by the electron irradiation or ion implantation contribute to the performance at higher temperatures.